No description available.

No description available.

The three-dimensional (3-D) structure of gp32*I, a major proteolytic fragment of the DNA helix-destabilizing protein from bacteriophage T4, has been determined at 18 A resolution by electron microscopy of negatively stained crystals and computer image analysis. The crystalline areas processed in 3-D have the symmetry of the space group P2₁, with a = 47 Å, b = 63 Å, c = 65 Å, and α = β = γ = 90°. This P2₁ unit cell contains one gp32*I molecule per asymmetric unit. The molecule is roughly V-shaped, containing two large domains linked by a smaller domain occupying the base of the V. The total length of the molecule is about 110 Å with an average diameter of about 25 Å. Systematic analysis of the symmetry in images of untilted crystals determined that the crystal could display several types of projection symmetry, pgg, pg corresponding to P2₁ symmetry with the screw axis along the a axis of the crystal, and pg corresponding to P2₁ symmetry with the screw axis along the b axis. Among images displaying pg symmetry along the b axis, two types of images with noticeably different appearances were obtained. A hypothesis was formed that explained the different types of symmetry as the result of the growth of the gp32*I crystal in the space group P2₁ 2₁ 2₁, in steps of 1/2 of a unit cell along the thin direction of the crystal. Two different types of 1/2 unit cell thick steps were postulated. Computer simulations were used to generate synthetic images of untilted crystals containing either one, two or three steps of each kind. The results of the simulations prove that the space group of the gp32*I crystal is P2₁ 2₁ 2₁. They suggest that careful analysis of the symmetry in images of untilted gp32*I crystals can provide information about the thickness of the crystals. A strategy is presented for determining the structure of the gp32*I crystal at higher resolution by electron microscopy of frozen, hydrated crystals. This strategy includes the use of symmetry analysis as a tool for determining the thickness of the crystals so that data from crystals of the same thickness can be combined in 3-D. A similar approach may prove useful in the 3-D electron microscopic analysis of other thin, multi-layered crystals.

Detectors for Electron Microscopy have traditionally used a scintillator to generate photons from fast electrons, which are then detected by a sensor. However, in recent years direct detection has become an area of interest due to the potential improvements to detector performance. In this thesis various aspects of direct detection are presented. I will begin with simulations of direct detectors based on Joy’s model of straight trajectories between Rutherford scattering events, where signal is generated by inelastic scattering events. The effects of microscope operating voltage, detector thickness, a surface electrically dead layer and diode depth on detector performance are presented. A prototype detector was developed using the DUOS sensor, two thicknesses of the sensor were produced a 50μm thick detector and a 20μm thick detector. EBSD results are presented which show how the use of a reactive ion etch to reduce the dead layer thickness of a mechanically thinned sensor improve the detection efficiency of a sensor allowing EBSD work to be carried out at operating voltages as low as 5keV. The MTF and DQE of both thicknesses of DUOS sensor are measured at 80kV and 200kV, which show that there is little difference between the two thicknesses at 80kV, but at 200kV the thinner detector shows an improved MTF. The results are then and compared with the equivalent simulated detectors. I show how the high frame rate of a detector and rigid and non-rigid registration can be used to improve image quality, resolving the {331} lattice spacing which is not visible with a simple summation of frames. Detectors using gallium nitride rather than silicon as the base semiconductor are simulated. The MTF at the Nyquist frequency for a GaN detector is double that of a Si detector at an operating voltages of 80kV due to the smaller interaction volume of an electron in GaN. However, at higher voltages the improvement is much smaller as most electrons pass through the detector.

No description available.

Abstract Diamond is nowadays regarded as a potential semiconductor material of the future, due to its extreme and unique properties. Some of these properties, in- clude its high hardness, highest breakdown ¯eld, high Debye temperature, high thermal conductivity, high hole and electron mobilities, large bandgap and op- tical transparency, among others. These properties make diamond suitable for high-temperature, high-speed and high-power electronic applicatons, as well as in other applications. However, defects associated with ion implantation have been shown to make it rather di±cult to obtain n-type doping in diamond. As such, an understanding of the nature of defects produced during ion implanta- tion of diamond remains a subject of great importance, if not essential, for the optimization of high-temperature, high-power electronic applications in partic- ular. In this respect, this study investigates the nature of the radiation damage generated within the collision cascades of multi-implantations of carbon ions in high-pressure, high-temperature single-crystal synthetic type Ib diamond, spread over a range of energies (50-150keV) and doses. This is achieved by means of the cold-implantation-rapid-annealing (CIRA) routine, and the anal- ysis of damage caused was done by using cross sectional transmission electron microscopy techniques. More precisely, the modes used to achieve this are the bright ¯eld transmission electron microscopy (BFTEM) coupled with selected area di®raction or SAD. At low dose implantation or at sub-critical implantation doses (2.5x1015 ions/cm2), it was found that the ion-damaged diamond layer consists of some threading dislocations, not homogeneously distributed which propagate from the surface into the ion-damaged diamond. In contrast to the sub-critical implantation doses , it was found that at very high implantation doses (7.0x1015 ions/cm2), i.e., above the critical dose (where diamond transforms to graphite upon annealing), the damaged diamond layer had some unconventional defect features close to the implanted surface.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, June, 2013 / Diamond with its outstanding and unique physical properties offers the opportunity to be used as semiconductor material in future device technologies. Promising ap- plications are, among others, high speed and high-power electronic devices working under extreme conditions, such as high temperature and harsh chemical environments. With respect to electronic applications, a controlled doping of the material is neces- sary which is preferably done by ion implantation. The ion implantation technique allows incorporation of foreign atoms at de¯ned depths and with controlled spatial distribution which is not achievable with other methods. However, the ion implanta- tion process is always connected with the formation of defects which compensate and trap charge carriers thus degrading the electrical behaviour. It is therefore essential to understand the nature of defects produced under various implantation conditions. In this respect, this study involves the investigation of the nature of the radiation damage produced during the multi-implantation of carbon ions in synthetic high- pressure, high-temperature (HPHT) type Ib diamond spread over a range of energies from 50 to 150 keV and °uences, using the cold-implantation-rapid-annealing (CIRA) routine. Single energy implantation of carbon ions in synthetic HPHT (type Ib), at room temperature, was also performed. Both ion milling and FIB (Focused Ion Beam) milling were used to prepare thin specimen for transmission electron micro- scope (TEM) analysis. The unimplanted, implanted and annealed samples were characterized using trans- mission electron microscopy based techniques and Raman spectroscopy. ii iii In unimplanted type Ia natural diamond, a high density of platelets, exhibiting the typical contrast of both edge-on and inclined platelets on f100g planes was found. As-implanted HPHT type Ib diamond, implanted with single energy of 150 keV car- bon ions and °uence of 7£1015 ions cm¡2 revealed an amorphous diamond layer of about 80 nm in thickness while, for low °uence implantations, the damaged diamond retained its crystallinity after annealing at 1600 K. In addition, damaged diamond transformed into disordered carbon comprising regions with bent (002) graphitic fringes and regions of amorphous carbon when high °uence, i.e., one above the amor- phization/graphitisation threshold were used followed by rapid thermal annealing at 1600 K. Furthermore, the interface between the implanted and annealed layer and the diamond substrate at the end of the range, showed diamond crystallites, inter- spersed between regions of amorphous carbon and partially graphitized carbon. This indicates that solid phase epitaxial recrystallization regrowth in diamond does not occur.

by Chan, Ray. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1994. / Includes bibliographical references (leaves [93-96]). / ACKNOWLEDGMENT / ABSTRACT / FIGURE CAPTIONS / TABLE CAPTIONS / INTRODUCTION / Chapter I. --- PRINCIPLES OF SEM / Chapter 1.1 --- STRUCTURE OF SEM --- p.1-1 / Chapter 1.2 --- IMAGE FORMATION --- p.1-6 / Chapter 1.2.1 --- Electron Beam-Specimen Interaction --- p.1-6 / Chapter 1.2.1.1 --- Electron scattering in solid specimen --- p.1-7 / Chapter 1.2.1.2 --- Electron range and spatial distribution --- p.1-7 / Chapter 1.2.1.3 --- Back scattered electrons(BE) --- p.1-8 / Chapter 1.2.1.4 --- Secondary electron(SE) --- p.1-9 / Chapter 1.2.1.5 --- Other signal types --- p.1-13 / Chapter 1.2.2 --- Types of Image Contrast --- p.1-14 / Chapter 1.3 --- DISTORTION AND NOISE --- p.1-17 / Chapter 1.3.1 --- Lens Aberration --- p.1-17 / Chapter 1.3.1.1 --- Spherical aberration --- p.1-17 / Chapter 1.3.1.2 --- Chromatic aberration --- p.1-18 / Chapter 1.3.1.3 --- Diffraction effect --- p.1-19 / Chapter 1.3.1.4 --- Axial astigmatism --- p.1-20 / Chapter 1.3.1.5 --- Spatial resolution calculation --- p.1-21 / Chapter 1.3.2 --- Image Defects --- p.1-22 / Chapter 1.3.2.1 --- Projection distortion --- p.1-22 / Chapter 1.3.2.2 --- Specimen tilting --- p.1-22 / Chapter 1.3.2.3 --- Moire effects --- p.1-22 / Chapter 1.3.3 --- Noise in SEM --- p.1-23 / Chapter II. --- SEM FOR IC TESTING / Chapter 2.1 --- QUANTITATIVE ANALYSIS OF VOLTAGE MEASUREMENT --- p.2-1 / Chapter 2.1.1 --- Energy Analysis of SEs --- p.2-1 / Chapter 2.1.2 --- Suppression of Local Fields --- p.2-2 / Chapter 2.1.3 --- "Elimination of Topography, Material Contrast and Work Function Variation" --- p.2-2 / Chapter 2.2 --- VOLTAGE RESOLUTION --- p.2-3 / Chapter 2.3 --- TIME RESOLUTION --- p.2-4 / Chapter 2.3.1 --- SE Generation Time --- p.2-4 / Chapter 2.3.2 --- SE Flight Time --- p.2-4 / Chapter 2.3.3 --- Required Voltage Resolution --- p.2-5 / Chapter 2.3.4 --- Electron Beam Pulse Width --- p.2-5 / Chapter 2.4 --- SPATIAL RESOLUTION --- p.2-5 / Chapter 2.5 --- CAPACITIVE COUPLING VOLTAGE CONTRAST --- p.2-6 / Chapter III. --- SEM TESTING TECHNIQUES / Chapter 3.1 --- CONVENTIONAL TESTING METHODS SYNOPSIS --- p.3-1 / Chapter 3.2 --- TESTING TECHNIQUES FOR SEM --- p.3-2 / Chapter 3.2.1 --- Static Mode --- p.3-2 / Chapter 3.2.2 --- Frequency Matching Mode --- p.3-3 / Chapter 3.2.2.1 --- Voltage coding --- p.3-4 / Chapter 3.2.2.2 --- Stroboscopy --- p.3-4 / Chapter 3.2.2.3 --- Waveform measurement --- p.3-6 / Chapter 3.2.2.4 --- Logic state mapping --- p.3-6 / Chapter 3.2.2.5 --- Frequency tracing --- p.3-7 / Chapter 3.2.2.6 --- Frequency mapping --- p.3-8 / Chapter 3.2.2.7 --- Logic state tracing --- p.3-9 / Chapter IV. --- CONVERT AMRAY 1830 INTO E-BEAM TESTER / Chapter 4.1 --- DESIGN CONSIDERATION --- p.4-1 / Chapter 4.1.1 --- Application Consideration --- p.4-1 / Chapter 4.1.2 --- Limitation of the AMRAY 1830 --- p.4-1 / Chapter 4.1.2.1 --- Detection system --- p.4-2 / Chapter 4.1.2.2 --- Scanning driver --- p.4-3 / Chapter 4.1.2.3 --- Beam blanker --- p.4-4 / Chapter 4.1.2.4 --- NibbleNet interface --- p.4-5 / Chapter 4.2 --- HARDWARE ARCHITECTURE --- p.4-6 / Chapter 4.2.1 --- SEM Circuit Varied --- p.4-7 / Chapter 4.2.1.1 --- Adding scanning relays --- p.4-7 / Chapter 4.2.1.2 --- Voltage clippers --- p.4-7 / Chapter 4.2.1.3 --- External scan interface in SEM --- p.4-9 / Chapter 4.2.2 --- PC Interface --- p.4-9 / Chapter 4.2.3 --- Driver Box --- p.4-10 / Chapter 4.2.3.1 --- Data preprocess unit --- p.4-10 / Chapter 4.2.3.2 --- Scanning preprocess unit --- p.4-12 / Chapter 4.2.3.3 --- Control unit --- p.4-12 / Chapter 4.2.4 --- Scanning Generation and Data Acquisition --- p.4-13 / Chapter 4.3 --- SOFTWARE DEVELOPED --- p.4-13 / Chapter 4.3.1 --- Function Library --- p.4-13 / Chapter 4.3.2 --- Integrated Environment --- p.4-14 / Chapter V. --- SYSTEM PERFORMANCE / Chapter 5.1 --- CHARACTERISTICS OF THE SCANNING DRIVERS --- p.5-1 / Chapter 5.1.1 --- Driver Output Changes with Acceleration Voltage --- p.5-2 / Chapter 5.1.2 --- Driver Output Changes with magnification --- p.5-3 / Chapter 5.1.3 --- Frequency Response --- p.5-4 / Chapter 5.1.4 --- Image Distortion --- p.5-7 / Chapter 5.2 --- INTEGRATED STUDY ENVIRONMENT --- p.5-9 / Chapter 5.2.1 --- Setting Status --- p.5-9 / Chapter 5.2.2 --- Image Scanning & Image Saving --- p.5-12 / Chapter 5.2.3 --- Static Probing --- p.5-15 / Chapter 5.2.4 --- Point Probing --- p.5-19 / Chapter 5.2.5 --- Frequency Matching --- p.5-20 / Chapter V. --- SUMMARY --- p.6-1 / REFERENCE / APPENDIX: / Chapter A. --- PROGRAM LISTING / Chapter B. --- CIRCUIT SCHEMATICS

No description available.

The morphology, composition, and distribution of elements within nanocrystals are critical parameters which dictate the material's properties and performance in a diverse array of emerging applications. The (scanning) transmission electron microscope ((S)TEM) represents a powerful tool for probing the structure and chemistry of materials on the nanoscale. Understanding of the mechanisms by which nanocrystals grow, transform, and degrade is vital if we are to develop rational synthesis routes and hence control the properties of the resulting materials. Electron microscopy represents a key tool in developing such an understanding. In situ techniques, where the material of interest is subjected to stimuli such as heat or a chemically reactive environment in the microscope, allow direct observation of dynamic transformations. Ex situ approaches, where multiple samples are prepared in the lab with the reaction parameters systematically altered, can also give important mechanistic insights. This thesis explores the use of both in situ and ex situ (S)TEM to gain insights into the growth and transformation of nanocrystals. Ex situ TEM is used to assess the structure of PbS nanocrystals in a polymer matrix, revealing new methods of morphological control through reaction temperature, precursor structures (appendix 4), and the processing of the polymer matrix (appendix 5). In situ techniques are used to observe the solution phase growth and shelling of nanocrystals (appendix 1) as well as the transformations of nanocrystals during heating in vacuum (appendices 2 and 3). The subjects of my in situ investigations are systems with heterogeneous distributions of elements. Historically, in situ electron microscope has been largely limited to imaging. However, to understand many dynamic transformations knowledge of changing elemental distributions is vital. For this reason, I have focused on the use of energy dispersive X-ray (EDX) spectroscopy to reveal changes in composition and elemental distributions during in situ experiments (appendices 1-3). This type of in situ elemental mapping is especially challenging for liquid-cell experiments, and my results represent the first report of EDX spectrum imaging for nanomaterials in liquid (appendix 1).